Lubricant base oil and lubricant composition

ABSTRACT

A lubricating base oil includes at least one of ionic liquids containing a compound represented by a formula (1): Z + A − , in which Z +  represents a cation and A −  represents an anion, in which Z +  is a cyclic quaternary ammonium ion having two different side chains and A −  is a conjugated amide ion.

TECHNICAL FIELD

The present invention relates to a lubricating base oil containing an ionic liquid, and a lubricating oil composition.

BACKGROUND ART

A lubricating oil generally includes an organic substance mainly containing hydrocarbons, where lowering the viscosity necessarily increases vapor pressure and entails evaporation loss of the lubricating oil and increase in risk of ignition. Especially, the lubricating oil (e.g. hydraulic oil) used in a facility such as machinery in an ironworks that handles high-temperature objects requires fire resistance in order to avoid fire accidents. Further, precision motors used for recent information equipment (e.g. hard disk) requires a lubricating oil exhibiting less evaporation and less scattering in order to minimize adverse effects on neighboring precision devices.

On the other hand, in recent years, it has been reported that an ionic liquid containing cations and anions exhibits an excellent thermal stability and a high ion conductivity and is stable even in the air (see, for instance, non-Patent Literature 1) Utilizing features of thermal stability (i.e., volatility resistance and fire resistance), high ion density (high ion conductivity), a large heat capacity, a low viscosity and the like of the ionic liquid, application of the ionic liquid has been vigorously studied on various usage such as an electrolyte for a solar cell (see, for instance, Patent Literature 1), an extraction separation solvent and a reaction solvent.

Moreover, use of such an ionic liquid as a lubricating base oil has been proposed (see, for instance, Patent Literature 2). Molecules of the ionic liquid are not linked by intermolecular attraction as in a molecular liquid but are linked by strong ionic bond. Accordingly, the ionic liquid is hardly volatile, exhibits fire resistance and stays stable against heat and oxidation. Thus, the ionic liquid is less evaporative even with a low viscosity and shows an excellent heat resistance.

CITATION LIST Patent Literature(s)

-   Patent Literature 1: JP-A-2003-31270 -   Patent Literature 2: International Publication WO2005/035702

Non-Patent Literature(s)

-   Non-Patent Literature 1: “J. Chem. Soc., Chem. Commun.” issued in     1992, p. 965

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Although an ionic liquid exhibits a low viscosity, a low vapor pressure and an excellent heat resistance as a lubricating base oil, an ionic liquid described in Example of Patent Literature 2 is insufficient since the ionic liquid is highly corrosive to metals under high-temperature environments. Thus, Patent Literature 2 does not clearly describe what cations and anions are most suitably selected as an ionic liquid used in a lubricating base oil.

Accordingly, in view of the above, an object of the invention is to provide a lubricating base oil exhibiting a low vapor pressure even with a low viscosity, no risk of ignition, an excellent heat resistance, less metal corrosion behavior at high temperatures and an excellent low-temperature fluidity, and a lubricating oil composition using the lubricating base oil.

Means for Solving the Problems

In order to solve the above-described problems, the invention provides a lubricating base oil and a lubricating oil composition as follows.

According to an aspect of the invention, a lubricating base oil includes at least one of ionic liquids containing a compound represented by a formula (1): Z⁺A⁻, in which Z⁺ represents a cation and A⁻ represents an anion, in which Z⁺ is a cyclic quaternary ammonium ion having two different side chains and A⁻ is a conjugated amide ion.

In the lubricating base oil according to the above aspect of the invention, it is preferable that A⁻ in the ionic liquid of the formula (1) is selected from anions having a structure represented by a formula (2) below.

In the formula (2), n is an integer of 1 to 4, m is an integer of 1 to 4, and n and m are allowed to be the same or different.

In the lubricating base oil according to the above aspect of the invention, it is preferable that Z⁺ in the ionic liquid of the formula (1) is selected from cations having a structure represented by a formula (3) below.

In the formula (3), n is 1 or 2; X is methylene or oxygen; and R₁ and R₂ each are a group selected from an alkyl group having 1 to 12 carbon atoms that is allowed to have an ether group, an ester group, a nitrite group and a silyl group.

In the lubricating base oil according to the above aspect of the invention, it is preferable that the ionic liquid has a molecular weight in a range of 410 to 570.

In the lubricating base oil according to the above aspect of the invention, it is preferable that the ionic liquid has a a kinematic viscosity at 40 degrees C. in a range of 1 mm²/s to 100 mm²/s.

In the lubricating base oil according to the above aspect of the invention, it is preferable that the ionic liquid has a pour point of at most zero degrees C.

According to another aspect of the invention, a lubricating oil composition containing: the lubricating base oil according to the above aspect of the invention; and at least one of an antioxidant, an oiliness agent, an extreme pressure agent, a detergent dispersant, a viscosity index improver, a rust inhibitor, a metal deactivator and an antifoaming agent.

It is preferable that the lubricant oil composition according to the another aspect of the invention is used for lubrication of an oil-impregnated bearing, a fluid dynamic bearing, vacuum equipment and semiconductor manufacturing equipment.

Accordingly, the invention can provide a lubricating base oil exhibiting a low vapor pressure even with a low viscosity, no risk of ignition, an excellent heat resistance, less metal corrosion behavior at high temperatures and an excellent low-temperature fluidity, and a lubricating oil composition using the lubricating base oil.

DESCRIPTION OF EMBODIMENT(S)

A lubricating base oil according to an exemplary embodiment includes at least one of later-described ionic liquids.

Each of the ionic liquids used in the exemplary embodiment is provided by an ionic liquid containing a compound represented by a formula (1): Z⁺A⁻, in which Z⁺ represents a cation and A⁻ represents an anion.

The ionic liquid requires Z⁺ to be a cyclic quaternary ammonium ion having two different side chains and A⁻ to be a conjugated amide ion in the formula (1).

It is preferable that A⁻ in the formula (1) is selected from anions having a structure represented by a formula (2) below.

In the formula (2), n is an integer of 1 to 4, preferably 1 or 2 in terms of a molecular weight of the ionic liquid. m is an integer of 1 to 4, preferably 1 or 2 in terms of the molecular weight of the ionic liquid. m and n may be mutually the same or different.

Examples of the anions represented by the formula (2) include bis(trifluoromethanesulfonyl)amide, bis(pentafluoroethanesulfonyl)amide, bis(heptafluoropropanesulfonyl)amide, bis(nonafluorobutanesulfonyl)amide, trifluoromethanesulfonyl(pentafluoroethanesulfonyl)amide, pentafluoroethanesulfonyl(heptafluoropropanesulfonyl)amide, heptafluoropropanesulfonyl(nonafluorobutanesulfonyl)amide, trifluoromethanesulfonyl(heptafluoropropanesulfonyl)amide, pentafluoroethanesulfonyl(nonafluorobutanesulfonyl)amide, trifluoromethanesulfonyl(nonafluorobutanesulfonyl)amide. Among these, in terms of the molecular weight of the ionic liquid, bis(trifluoromethanesulfonyl)amide, bis(pentafluoroethanesulfonyl)amide, and trifluoromethanesulfonyl(pentafluoroethanesulfonyl)amide are preferable, among which bis(trifluoromethanesulfonyl)amide is particularly preferable.

It is preferable that Z⁺ in the formula (1) is selected from cations having a structure represented by a formula (3) below.

In the formula (3), n is 1 or 2 and X is methylene or oxygen. R₁ and R₂ each are a group selected from an alkyl group having 1 to 12 carbon atoms that may have an ether group (ether bond), an ester group (ester bond), a nitrile group and a silyl group. The number of carbon atoms in such an alkyl group is more preferably 1 to 6, particularly preferably 1 to 4 in terms of reduction in the viscosity and improvement in heat resistance (high-temperature oxidation stability) of the ionic liquid.

Examples of the cations represented by the formula (3) include 1-butyl-1-methylpyrrolidinium, 1-pentyl-1-methylpyrrolidinium, 1-hexyl-1-methylpyrrolidinium, 1-heptyl-1-methylpyrrolidinium, 1-octyl-1-methylpyrrolidinium, 1-nonyl-1-methylpyrrolidinium, 1-decyl-1-methylpyrrolidinium, 1-undecyl-1-methylpyrrolidinium, 1-dodecyl-1-methylpyrrolidinium, 1-(2-methoxyethyl)-1-methylpyrrolidinium, 1-(2-methoxy-2-oxoethyl)-1-methylpyrrolidinium, 1-cyanomethyl-1-methylpyrrolidinium, 1-trimethylsilylmethyl-1-methylpyrrolidinium, 1-butyl-1-methylpiperidinium, 1-pentyl-1-methylpiperidinium, 1-hexyl-1-methylpiperidinium, 1-heptyl-1-methylpiperidinium, 1-octyl-1-methylpiperidinium, 1-nonyl-1-methylpiperidinium, 1-decyl-1-methylpiperidinium, 1-undecyl-1-methylpiperidinium, 1-dodecyl-1-methylpiperidinium, 1-(2-methoxyethyl)-1-methylpiperidinium, 1-(2-methoxy-2-oxoethyl)-1-methylpiperidinium, 1-cyanomethyl-1-methylpiperidinium, 1-trimethylsilylmethyl-1-methylpiperidinium, 1-butyl-1-methylmorpholinium, 1-pentyl-1-methylmorpholinium, 1-hexyl-1-methylmorpholinium, 1-heptyl-1-methylmorpholinium, 1-octyl-1-methylmorpholinium, 1-nonyl-1-methylmorpholinium, 1-decyl-1-methylmorpholinium, 1-undecyl-1-methylmorpholinium, 1-dodecyl-1-methylmorpholinium, 1-(2-methoxyethyl)-1-methylmorpholinium, 1-(2-methoxy-2-oxoethyl)-1-methylmorpholinium, 1-cyanomethyl-1-methylmorpholinium and 1-trimethylsilylmethyl-1-methylmorpholinium. Among the above, in terms of reducing the viscosity and improvement in the heat resistance (high-temperature oxidation stability) of the ionic liquid, 1-butyl-1-methylpyrrolidinium, 1-pentyl-1-methylpyrrolidinium, 1-hexyl-1-methylpyrrolidinium, 1-(2-methoxyethyl)-1-methylpyrrolidinium, 1-butyl-1-methylpiperidinium, 1-(2-methoxyethyl)-1-methylpiperidinium and 1-(2-methoxyethyl)-1-methylmorpholinium are preferable, among which 1-butyl-1-methylpyrrolidinium, 1-(2-methoxyethyl)-1-methylpyrrolidinium and 1-(2-methoxyethyl)-1-methylpiperidinium are particularly preferable.

A molecular weight of the ionic liquid is preferably from 410 to 570, more preferably from 410 to 470, particularly preferably from 420 to 440. When the molecular weight falls within the above range, a charge density and an alkyl chain of the cations range appropriately, thereby reducing the viscosity and improving the heat resistance (high-temperature oxidation stability) of the ionic liquid.

In order to restrain power loss caused by evaporation loss and viscosity resistance, the kinematic viscosity of the ionic liquid at 40 degrees C. is preferably in a range of 1 mm²/s to 100 mm²/s, more preferably of 10 mm²/s to 70 mm²/s, particularly preferably of 20 mm²/s to 40 mm²/s.

In order to restrain the increase in the viscosity resistance at low temperatures, the pour point of the ionic liquid is preferably at most zero degrees C., more preferably at most −10 degrees C., particularly preferably at most −20 degrees C.

In order to avoid corrosion on to-be-lubricated component, the acid value of the ionic liquid is preferably at most 1 mgKOH/g, more preferably at most 0.5 mgKOH/g, particularly preferably at most 0.3 mgKOH/g.

In order to reduce evaporation of the base oil, the flash point of the ionic liquid is preferably at least 200 degrees C., more preferably at least 250 degrees C., particularly preferably at least 300 degrees C.

In order to avoid excessive change in viscosity according to the temperature, the viscosity index of the ionic liquid is preferably at least 80, more preferably at least 100, particularly preferably at least 120.

An ion concentration (measured at 20 degrees C.) of the ionic liquid is preferably at least 1 mol/dm³, more preferably at least 1.5 mol/dm³, particularly preferably at least 2 mol/dm³. The ion concentration of the ionic liquid is calculated by [density (g/cm³)/molecular weight Mw (g/mol)]×1000. When the ion concentration of the ionic liquid is less than 1 mol/dm³, low evaporativity and heat-resistance (i.e., features of the ionic liquid) are disadvantageously lowered.

Although the lubricating base oil in the exemplary embodiment contains at least one of the aforementioned ionic liquid, the lubricating base oil in the exemplary embodiment may contain other compositions (e.g., ethyl acetate) in addition to the ionic liquid. However, in order to be advantageous as the lubricating base oil in the exemplary embodiment, a ratio of the ionic liquid in the lubricating base oil is preferably at least 50 mass %, more preferably at least 70 mass %, further preferably at least 90 mass %, particularly preferably 100 mass %.

The lubricating base oil in the exemplary embodiment is usable for a variety of applications by containing a predetermined additive. Examples of the additive include an antioxidant, an oiliness agent, an extreme pressure agent, a detergent dispersant, a viscosity index improver, a rust inhibitor, a metal deactivator and an antifoaming agent. One of the additives may be solely used or at least two of the additives may be used in combination. It should be noted that the lubricating base oil may be directly used without containing the additives depending on usage.

Examples of the antioxidant include an aminic antioxidant, a phenolic antioxidant, a phosphorous antioxidant and a sulfuric antioxidant. One of the antioxidants may be solely used or at least two of the antioxidants may be used in combination. Examples of the aminic antioxidant include: monoalkyldiphenylamine compounds such as monooctyldiphenylamine and monononyldiphenylamine; dialkyldiphenylamine compounds such as 4,4′-dibutyldiphenylamine, 4,4′-dipentyldiphenylamine, 4,4′-dihexyldiphenylamine, 4,4′-diheptyldiphenylamine, 4,4′-dioctyldiphenylamine and 4,4′-dinonyldiphenylamine; polyalkyldiphenylamine compounds such as tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine and tetranonyldiphenylamine; and naphthylamine compounds such as alpha-naphthylamine, phenyl-alpha-naphthylamine, butylphenyl-alpha-naphthylamine, pentylphenyl-alpha-naphthylamine, hexylphenyl-alpha-naphthylamine, heptylphenyl-alpha-naphthylamine, octylphenyl-alpha-naphthylamine and nonylphenyl-alpha-naphthylamine.

Examples of the phenolic antioxidant include: monophenolic compounds such as 2,6-di-tert-butyl-4-methylphenol and 2,6-di-tert-butyl-4-ethylphenol; and diphenolic compounds such as 4,4′-methylenebis(2,6-di-tert-butylphenol) and 2,2′-methylenebis(4-ethyl-6-tert-butylphenol).

Examples of the sulfuric antioxidant include: thioterpene compounds such as 2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazine-2-ylamino)phenol and a reactant of phosphorus pentasulfide and pinene; and dialkyl thiodipropionate such as dilauryl thiodipropionate and distearyl thiodipropionate.

Examples of the phosphorous antioxidant include triphenyl phosphite and diethyl [3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]phosphonate.

A content of the antioxidant is typically in a range of 0.01 mass % to 10 mass % based on a total amount of the lubricating oil, preferably in a range of 0.03 mass % to 5 mass %.

Examples of the oiliness agent include: aliphatic alcohol; fatty acid compounds such as fatty acid and fatty acid metal salt; ester compounds such as polyol ester, sorbitan ester and glyceride; and amine compounds such as aliphatic amine. In view of blending effects, a content of the oiliness agent is typically in a range of 0.1 mass % to 30 mass % based on the total amount of the lubricating oil, preferably in a range of 0.5 mass % to 10 mass %.

Examples of the extreme pressure agent include a sulfuric extreme pressure agent, a phosphorus extreme pressure agent, an extreme pressure agent containing sulfur and metal and an extreme pressure agent containing phosphorous and metal. One of the extreme pressure agents may be solely used or at least two of the extreme pressure agents may be used in combination. Any extreme pressure agent may be used as long as the extreme pressure agent contains at least one of a sulfur atom and a phosphorous atom in a molecule and exhibits load bearing and wear resistance. Examples of the extreme pressure agent containing sulfur in the molecule include sulfurized fat and oil, sulfurized fatty acid, ester sulfide, olefin sulfide, dihydrocarbyl polysulfide, a thiadiazole compound, an alkylthiocarbamoyl compound, a triazine compound, a thioterpene compound and a dialkyl thiodipropionate compound.

Examples of the extreme pressure agent containing sulfur, phosphorous and metal include: zinc dialkylthiocarbamate (Zn-DTC), molybdenum dialkylthiocarbamate (Mo-DTC), lead dialkylthiocarbamate, tin dialkylthiocarbamate, zinc dialkyldithiocarbamate (Zn-DTP), molybdenum dialkyldithiophosphate (Mo-DTP), sodium sulfonate and calcium sulfonate. Representative examples of the extreme pressure agent containing phosphorous in the molecule include phosphate such as tricresyl phosphate, and an amine salt thereof. In view of the blending effects and economical efficiency, a content of the extreme pressure agent is typically in a range of 0.01 mass % to 30 mass % based on the total amount of the lubricating oil, preferably in a range of 0.01 mass % to 10 mass %.

Examples of the detergent dispersant include metal sulfonates, metal salicylates, metal phenates and succinimides. In view of the blending effects, a content of the detergent dispersant is typically in a range of 0.1 mass % to 30 mass % based on the total amount of the lubricating oil, preferably in a range of 0.5 mass % to 10 mass %.

Examples of the viscosity index improver include polymethacrylate, dispersive polymethacrylate, an olefin copolymer (e.g. an ethylene-propylene copolymer), a dispersive olefin copolymer and a styrene copolymer (e.g. a styrene-diene copolymer hydride). In view of the blending effects, a content of the viscosity index improver is typically in a range of 0.5 mass % to 35 mass % based on the total amount of the lubricating oil, preferably in a range of 1 mass % to 15 mass %.

Examples of the rust inhibitor include metal sulfonates, succinates and alkanolamines such as alkylamines and monoisopropanolamines. In view of the blending effects, a content of the rust inhibitor is typically in a range of 0.01 mass % to 10 mass % based on the total amount of the lubricating oil, preferably in a range of 0.05 mass % to 5 mass %.

Examples of the metal deactivator include benzotriazole and thiadiazole. In view of the blending effects, a content of the metal deactivator is typically in a range of 0.01 mass % to 10 mass % based on the total amount of the lubricating oil, preferably in a range of 0.01 mass % to 1 mass %.

Examples of the antifoaming agent include methyl silicone oil, fluorosilicone oil and polyacrylates. In view of the blending effects, a content of the antifoaming agent is typically about 0.0005 mass % to 0.01 mass % based on the total amount of the lubricating oil.

In order to avoid lowering of viscosity and corrosion, a water content in a lubricating oil composition in the exemplary embodiment is preferably at most 3000 mass ppm based on the composition, more preferably at most 500 mass ppm, particularly preferably at most 100 mass ppm.

The lubricating base oil in the exemplary embodiment exhibits an extremely low metal corrosion behavior, a low vapor pressure even with a low viscosity and no risk of ignition. Accordingly, the lubricating base oil can be applied to various fields in a direct manner or as the lubricating oil composition added with the above additives.

For instance, the lubricating base oil can be preferably used for: an internal combustion engine; a torque transmitter such as fluid coupling, automatic transmission (AT) and continuously-variable transmission (CVT); a bearing (e.g. slide bearing, ball bearing, oil-impregnated or oil-retaining bearing, fluid dynamic bearing); a compressor; a chain; a gear; a hydraulic device; a vacuum pump; a timepiece component; a hard disc; an aerospace instrument such as airplane and artificial satellite; a sealing instrument; a motor; and the like. The lubricating base oil can also be applied to a rolling device such as a ball screw and a rolling guide surface, a clutch control rotation transmitter, a power-steering device, a reciprocating compressor and a turbocharger.

Further, the lubricating base oil in the exemplary embodiment can also be suitable as a metalworking fluid (cutting, pressing and forging), a mold releasing agent, a heat-treating agent, a heat medium, a cooling agent, a rust inhibitor, a buffering agent such as a damper and a conductive lubricating agent that requires conductivity.

The lubricating base oil in the exemplary embodiment is also applicable as a base oil of grease. Examples of a thickener of grease include: a metal soap thickener such as a lithium salt and a calcium salt; and a non-metal thickener. Examples of the non-metal thickener includes, for instance, bentonite, silica and fluorine resin powder. The lubricating base oil in the exemplary embodiment is also applicable as a base oil of gelatinous material other than grease.

Further, the invention is suitable as a machinery material containing iron, copper, aluminum and zinc. The invention is especially suitable when known corrosion-resistant materials such as stainless steel (martensite, ferrite, austenite), ceramic material (e.g. silicon nitride (Si₃N₄), silicon carbide (SiC), alumina (Al₂O₃), aluminum nitride (AlN), boron carbide (B₄C), titanium boride (TiB₂), boron nitride (BN), titanium carbide (TiC), titanium nitride (TiN), zirconia (ZrO₂) and the like are used and when a material on which various coating processing is conducted by DLC (diamond-like-carbon) processing is used.

Further, the invention is also suitably used in a device for conducting physical vapor deposition (PVD) or chemical vapor deposition (CVD). Examples of the physical vapor deposition include vacuum deposition, sputtering, ion plating and ion implantation using various ion guns. Examples of the vacuum deposition include electron beam evaporation, ion-assisted electron beam evaporation, arc evaporation and the like as well as general resistance heating evaporation. The physical evaporation may be conducted in combination. Examples of the CVD include thermal CVD, plasma CVD, optical CVD, epitaxial CVD, and atomic-layer CVD. The chemical vapor deposition may be used in combination or, alternatively, may be used in combination with physical vapor deposition.

The PVD device and CVD device using the lubricating base oil (or the lubricating oil composition) in the exemplary embodiment is, for instance, suitably used for manufacturing a display element.

EXAMPLES

Next, the invention will be further described in detail based on Examples and Comparatives, which by no means limit the invention. It should be noted that the properties (i.e., kinematic viscosity, viscosity index, pour point, 5%-mass reducing temperature, friction property and metal corrosion behavior) of the lubricating base oil and the lubricating oil composition were evaluated or measured according to the following methods.

(1) Kinematic Viscosity

A kinematic viscosity was measured according to “Crude petroleum and petroleum products-Determination of kinematic viscosity and calculation of viscosity index from kinematic viscosity” defined in JIS (Japanese Industrial Standards) K2283.

(2) Viscosity Index

A viscosity index was measured according to “Crude petroleum and petroleum products-Determination of kinematic viscosity and calculation of viscosity index from kinematic viscosity” defined in JIS K2283.

(3) Pour Point

A pour point was measured according to the method described in JIS K2269.

(4) 5%-Mass Reducing Temperature

Using a differential thermal analyzer, the temperature was raised at a rate of 10 degrees C./min and the temperature at which initial mass was reduced by 5% was measured. Higher 5%-mass reducing temperature indicates more excellent evaporation resistance and heat-resistance.

(5) Friction Property (Friction Coefficient and Wear Width)

Using a ball-on-disc type reciprocating friction tester (Bowden-Leben type), a friction coefficient and a wear width were measured under conditions of 20N-load, 80-degree-C. temperature, 30-mm²/s sliding velocity and 15-mm stroke. The ball is made of a material denoted by SUJ2 and has a 10-mm diameter. The disc is made of a material denoted by SUJ2. Lower friction coefficient and wear width indicate more excellent lubricity and wear resistance.

(6) Metal Corrosion Behavior

One of iron sintered bearings (having an iron content of at least 99 mass %) and one of copper sintered bearings (having a copper content of 93 mass % to 98 mass %, a tin content of 2 mass % to 7 mass %, and a content of other metals of at most 1 mass %) were simultaneously soaked in an 8-mL ionic liquid and left still for 240 hours at 140 degrees C. Then, an appearance of the ionic liquid was observed. Each of the bearings has 12-mm outer diameter and a 4-mm thickness. Metal corrosion behavior was evaluated according to the following criteria:

-   -   A: Neither metal elution nor corrosion was observed.     -   B: A brownish or black eluted substance was slightly observed         (the substance was slightly corroded).     -   C: A brownish or black eluted substance was observed (the         substance was corroded).

Ionic Liquids

The following ionic liquids were synthesized or prepared as follows.

(1) Ionic Liquid 1: 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide

1-methylpyrrolidine (50 g, 0.585 mol) and 2-propanol (70 mL) were added to a 1-L flask under a nitrogen atmosphere. After 1-bromobutane (96.5 g, 0.705 mol) was dropped therein, the mixture was raised to 40 degrees C. and was reacted for six hours. After completion of the reaction, the mixture was recrystallized with ethyl acetate and subjected to filtration. The obtained crystals were washed for several times with ethyl acetate. Subsequently, the crystals were dried at 40 degrees C. for several hours under reduced pressure using a vacuum pump to obtain 1-butyl-1-methylpyrrolidinium bromide (halogen body) (113 g, 0.510 mol).

Next, the halogen body (113 g, 0.510 mol) and deionized water (110 mL) were put into the 1-L flask, into which an aqueous solution prepared by dissolving lithium bis(trifluoromethanesulfonyl)imide (151 g, 0.525 mol) in deionized water (150 mL) was dropped. After being stirred for about one hour at the room temperature, the reaction mixture was transferred to a 1-L separating funnel and was extracted with methylene chloride (230 mL). The collected methylene chloride solution was washed for several times with deionized water. After washing, an aqueous layer (in a range of 1 mL to 2 mL) was collected and reacted with a 0.5M silver nitrate solution (1 mL), where the presence of precipitates was checked. (If white precipitates were observed, since bromide ions were not completely removed, washing is repeated until the precipitates are not confirmed.) After completion of water-washing, the reaction mixture was condensed using a rotary evaporator, to which a little amount of activated carbon was added and stirred for one day at the room temperature. The mixture was put into a column of a neutral alumina and was heated at 60 degrees C. for four hours with stirring under reduced pressure using a vacuum pump to obtain a target compound (212 g, 0.50 mol). A chemical formula of the compound is shown below.

(2) Ionic Liquid 2: 1-hexyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide

Except that 1-bromohexane (116 g, 0.705 mol) was used instead of 1-bromobutane, the same process as in synthesizing the ionic liquid 1 was conducted to obtain 1-hexyl-1-methylpyrrolidinium bromide (117 g, 0.468 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (202 g, 0.449 mol). A chemical formula of the compound is shown below.

(3) Ionic Liquid 3: 1-(2-methoxyethyl)-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide

Except that 2-iodoethylmethylether (131 g, 0.705 mol) was used instead of 1-bromobutane, the same process as in synthesizing the ionic liquid 1 was conducted to obtain 1-(2-methoxyethyl)-1-methylpyrrolidinium iodide (146 g, 0.538 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (212 g, 0.500 mol). A chemical formula of the compound is shown below.

(4) Ionic Liquid 4: 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)amide

Except that 1-methylpiperidine (58 g, 0.585 mol) was used instead of 1-methylpyrrolidine and the reaction temperature was 80 degrees C., the same process as in synthesizing the ionic liquid 1 was conducted to obtain 1-butyl-1-methylpiperidinium bromide (137 g, 0.579 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (250 g, 0.573 mol). A chemical formula of the compound is shown below.

(5) Ionic Liquid 5: 1-(2-methoxyethyl)-1-methylpiperidinium bis(trifluoromethanesulfonyl)amide

Except that 1-methylpiperidine (58 g, 0.585 mol) was used instead of 1-methylpyrrolidine and the reaction temperature was 60 degrees C., the same process in synthesizing the ionic liquid 3 was conducted to obtain 1-(2-methoxyethyl)-1-methylpiperidinium iodide (161 g, 0.563 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (241 g, 0.549 mol). A chemical formula of the compound is shown below.

(6) Ionic Liquid 6: 1-(2-methoxyethyl)-1-methylmorpholinium bis(trifluoromethanesulfonyl)amide

Except that 1-methylmorpholine (59 g, 0.585 mol) was used instead of 1-methylpiperidine and the reaction temperature was 80 degrees C., the same process as in synthesizing the ionic liquid 3 was conducted to obtain 1-(2-methoxyethyl)-1-methylmorpholinium iodide (145 g, 0.505 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (202 g, 0.460 mol). A chemical formula of the compound is shown below.

(7) Ionic Liquid 7: 1-butylpyridinium bis(trifluoromethanesulfonyl)amide

Except that pyridine (46 g, 0.585 mol) was used instead of 1-methylpyrrolidine and acetonitrile (200 mL) was used instead of 2-propanol and the reaction temperature was 80 degrees C., the same process as in synthesizing the ionic liquid 1 was conducted to obtain 1-butylpyridinium bromide (125 g, 0.579 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (234 g, 0.562 mol). A chemical formula of the compound is shown below.

(8) Ionic Liquid 8: N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide

The ionic liquid 8 was purchased from KANTO CHEMICAL CO., INC. A chemical formula of the compound is shown below.

(9) Ionic Liquid 9: N,N,N-trimethyl-N-pentyl ammonium bis(trifluoromethanesulfonyl)amide

A 3.2M solution of trimethylamine in methanol (183 mL, 0.585 mol) and 1-iodopentane were added to a 1-L flask under a nitrogen atmosphere and reacted for several hours at the room temperature. After completion of the reaction, the solvent was removed under reduced pressure. A residue was washed several times with ethyl acetate and acetonitrile. Subsequently, the residue was dried for several hours under reduced pressure using a vacuum pump to obtain N,N,N-trimethyl-N-pentyl ammonium iodide (89 g, 0.346 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (138 g, 0.336 mol). A chemical formula of the compound is shown below.

(10) Ionic Liquid 10: triethyl(octyl)phosphonium bis(trifluoromethanesulfonyl)amide

A 20% solution of triethylphosphine in toluene (346 g, 0.585 mol) and 1-iodooctane (211 g, 0.878 mol) were added to a 1-L flask under a nitrogen atmosphere and reacted at 60 degrees C. for several hours. After completion of the reaction, the mixture was washed for several times with ethyl acetate and dried at 40 degrees C. for several hours under reduced pressure using a vacuum pump to obtain triethyl(octyl)phosphonium iodide (176 g, 0.491 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (241 g, 0.471 mol). A chemical formula of the compound is shown below.

(11) Ionic Liquid 11: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide

1-methylimidazole (173 g, 2.100 mol) and 1-chlorobutane (234 g, 2.528 mol) were added to a 1-L flask under a nitrogen atmosphere and reacted at 90 degrees C. for several hours. After completion of the reaction, the reaction mixture was recrystallized with ethyl acetate and acetonitrile and subjected to filtration. The obtained crystals were washed at 40 degrees C. for several times under reduced pressure using a vacuum pump to obtain 1-butyl-1-methylimidazolium chloride (352 g, 2.016 mol). Except that this quaternary salt was used instead of 1-butyl-1-methylpyrrolidinium bromide, the same process as in synthesizing the ionic liquid 1 was conducted to obtain a target compound (837 g, 1.996 mol). A chemical formula of the compound is shown below.

(12) Ionic Liquid 12: 1-butyl-1-methylpyrrolidinium trifluorotris(pentafluoroethyl)phosphate

The ionic liquid 12 was purchased from Merck Ltd. A chemical formula of the compound is shown below.

(13) Ionic Liquid 13: 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate

1-butylpyrrolidine (34 g, 0.267 mol) and toluene (230 mL) were added to a 1-L flask under a nitrogen atmosphere. A mixture solution of methyltriflate (43 g, 0.262 mol) and toluene (100 mL) was dropped into the flask at zero degrees C. and reacted at the same temperature for 24 hours. After completion of the reaction, the reaction mixture was washed several times with toluene and treated with activated carbon. The reaction mixture was put into a column of a neutral alumina and was heated at 60 degrees C. for four hours with stirring under reduced pressure using a vacuum pump to obtain a target compound (68 g, 0.233 mol). A chemical formula of the compound is shown below.

Examples 1 to 8 and Comparatives 1 to 7

Using the above ionic liquids and the following additives, according to formulation shown in Tables 1 and 2, lubricating base oils or lubricating oil compositions were prepared. The above properties (i.e., kinematic viscosity, viscosity index, pour point, 5%-mass reducing temperature, friction property and metal corrosion behavior) of the lubricating base oils and the lubricating oil compositions were evaluated or measured. The results are shown in Tables 1 and 2 together with the formulation.

Extreme pressure agent: tricresyl phosphate Metal deactivator: benzotriazole

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 formulation base oil ionic liquid 1 100 99.9 98.9 (mass %) ionic liquid 2 100 ionic liquid 3 100 ionic liquid 4 100 ionic liquid 5 100 ionic liquid 6 100 additive extreme pressure agent — — — — — — — 1 metal deactivator — — — — — — 0.1 0.1 evaluation items base oil molecular weight 422 450 424 436 438 440 422 422 kinematic viscosity at 40° C. 30.06 42.33 21.34 61.20 35.72 80.79 29.85 30.19 (mm²/s) kinematic viscosity at 100° C. 6.364 7.696 5.170 8.889 6.548 9.535 6.315 6.330 (mm²/s) viscosity index 171 153 187 121 139 94 170 168 pour point (° C.) <−50 −15 <−50 −40 <−50 −40 −40 −40 5%-mass reducing temp. 380 372 388 383 379 366 377 378 (° C.) corrosion behavior A A A A A A A A friction coefficient 0.079 0.077 0.082 0.089 0.089 0.084 0.086 0.082 wear width (mm) 0.32 0.28 0.28 0.24 0.32 0.23 0.11 0.10

TABLE 2 Comparative Comparative Comparative Comparative Comparative Comparative Comparative 1 2 3 4 5 6 7 formulation base oil ionic liquid 7 100 (mass %) ionic liquid 8 100 ionic liquid 9 100 ionic liquid 10 100 ionic liquid 11 100 ionic liquid 12 100 ionic liquid 13 100 additive extreme pressure agent — — — — — — — metal deactivator — — — — — — — evaluation items base oil molecular weight 416 426 410 512 419 587 291 kinematic viscosity at 22.86 26.45 45.15 45.79 19.96 61.00 65.08 40° C. (mm²/s) kinematic viscosity at 5.117 5.450 7.566 7.675 4.732 8.662 10.705 100° C. (mm²/s) viscosity index 162 148 134 135 166 115 155 pour point (° C.) 26 −50 25 <−50 <−50 4 3 5%-mass reducing temp. 365 365 356 356 397 325 367 (° C.) corrosion behavior B B A C C C B friction coefficient 0.098 0.086 0.072 0.068 0.093 0.072 0.048 wear width (mm) 0.31 0.30 0.27 0.24 0.34 <0.01 0.21

As clearly recognized from the evaluation results shown in Tables 1 and 2, the lubricating base oils or the lubricating oil compositions obtained in Examples 1 to 8 exhibited favorable heat resistance and lubricity in addition to less metal corrosion behavior at higher temperatures and an excellent low-temperature fluidity. On the other hand, it was recognized that, when using an ionic liquid not satisfying that the cation was a cyclic quaternary ammonium ion having two different side chains and the anion was a conjugated amide ion, less metal corrosion behavior at higher temperatures and an excellent low-temperature fluidity were not simultaneously achieved although heat resistance and lubricity were excellent, so that the obtained compound was not appropriate as the lubricating base oil. 

1. A lubricating base oil comprising an ionic liquid comprising a compound of formula (1): Z⁺A⁻  (1) wherein Z⁺ is a cyclic quaternary ammonium cation having two different side chains and A⁻ is a conjugated amide anion.
 2. The lubricating base oil of claim 1, wherein A⁻ is of formula (2):

n is an integer of from 1 to 4, m is an integer of from 1 to 4, and n and m are allowed to be the same or different.
 3. The lubricating base oil of claim 1, wherein Z⁺ is of formula (3):

n is 1 or 2; X is methylene or oxygen; and each R₁ and R₂ is independently an alkyl group having from 1 to 12 carbon atoms, optionally comprising an ether group, an ester group, a nitrile group and a silyl group.
 4. The lubricating base oil of claim 1, wherein the ionic liquid has a molecular weight of from 410 to
 570. 5. The lubricating base oil of claim 1, wherein the ionic liquid has a kinematic viscosity at of from 1 mm²/s to 100 mm²/s at 40 degrees C.
 6. The lubricating base oil of claim 1, wherein the ionic liquid has a pour point of at most zero degrees C.
 7. A lubricating oil composition comprising: the lubricating base oil of claim 1; and at least one selected from the group consisting of an antioxidant, an oiliness agent, an extreme pressure agent, a detergent dispersant, a viscosity index improver, a rust inhibitor, a metal deactivator and an antifoaming agent.
 8. A method of lubricating an object, the method comprising: contacting the lubricating oil composition of claim 7 with an object in need thereof, wherein the object is at least one selected from the group consisting of an oil-impregnated bearing, a fluid dynamic bearing, vacuum equipment, and semiconductor manufacturing equipment.
 9. The lubricating base oil of claim 2, wherein n is an integer of from 1 to 2, m is an integer of from 1 to 2, and n and m are allowed to be the same or different.
 10. The lubricating base oil of claim 3, wherein the alkyl group further comprises at least one selected from the group consisting of an ether group, an ester group, a nitrile group, and a silyl group.
 11. The lubricating base oil of claim 3, wherein the alkyl group has from 1 to 6 carbon atoms.
 12. The lubricating base oil of claim 3, wherein the alkyl group has from 1 to 4 carbon atoms.
 13. The lubricating base oil of claim 4, wherein the ionic liquid has a molecular weight of from 410 to
 470. 14. The lubricating base oil of claim 4, wherein the ionic liquid has a molecular weight of from 420 to
 440. 15. The lubricating base oil of claim 5, wherein the ionic liquid has a kinematic viscosity of from 10 mm²/s to 70 mm²/s at 40 degrees C.
 16. The lubricating base oil of claim 5, wherein the ionic liquid has a kinematic viscosity of from 20 mm²/s to 40 mm²/s at 40 degrees C.
 17. The lubricating base oil of claim 6, wherein the ionic liquid has a pour point of at most −10 degrees C.
 18. The lubricating base oil of claim 6, wherein the ionic liquid has a pour point of at most −20 degrees C.
 19. The lubricating base oil of claim 1, wherein the ionic liquid has an acid value of at most 1 mgKOH/g.
 20. The lubricating base oil of claim 1, wherein the ionic liquid has a flash point of at least 200 degrees C. 